Computing systems used in data centers and other environments where reliability and availability is an important performance criteria often use redundant components or modules, so that a failure of any single component or module does not substantially compromise the performance of the system or result in data loss.
For stored data, the desired system reliability characteristics may be achieved by storing duplicate or redundant copies of the data on independently operated memory systems. Active data may be stored, for example, using a RAID concept, where the where a chunk of data is distributed over a plurality of memory modules, and a parity or other error correcting code, is computed from the data being protected by the parity and stored on another memory module. Data may be stored at a plurality of physical locations as well.
To cope with hardware failures (e.g., electronics, fans, prime power), redundant components and power sources may be provided, and the components may transfer a function to the redundant backup component automatically. Moreover, in some systems, a failed component or module may be designed such that the component or module, for example, a circuit card or power supply, may be replaced without interrupting the operation of the system. More often, the system needs to be scheduled for down time so that the failed component may be replaced. Taking a high performance system off line to effect a repair may be problematic, as the system may contain a large amount of data that needs to be continually and rapidly accessed. Systems needing off-line replacement of components thus suffer from a lower availability and may compromise the promised levels of service that is to be provided (e.g., service level agreements, SLA).
Alternatively, redundant electronic modules, for example, may be provided, and such modules may be used to continue the operation of the system. Modules may be designed to “hot swappable”, where this term is intended to convey the concept that a failed module may be removed from a larger system and replaced with a working module without taking the system off line to perform the replacement A redundant module may maintain the operation during this interval. The computing system may need to respond to the failure or prepare for such a repair action by providing automatically or manually configured temporary storage for new data, reconstructing lost data using the RAIDed data, connecting a standby component, or the like. From a physical and electrical viewpoint, the failed component or module needs to be capable of being removed from the system and replaced with a working module without causing artifacts on a data bus, causing electrical transients associated with grounding and other connections being made or broken during the removal of a module and the insertion of another module.
In particular, one often arranges that the ground pin on a connector is longer than the power pin of the connector, so that the power, being supplied from a common bus on the motherboard is disconnected from the module before the ground connection is interrupted by withdrawing the module form a mating connector. This same arrangement causes the ground connection to be re-established before power is supplied to the module upon insertion into the connector.
When a module is removed from a chassis, the mating connector on the motherboard or other interface remains connected to the power distribution bus and has power applied thereto. An inadvertent short circuit could cause damage to the power supply and shut down the entire system. Most systems that can be serviced while in operation are configured so that the plane of the modules is vertical and that the direction of motion of the insertion/withdrawal involves horizontal motion. As such the connector is recessed into the chassis and typically has a circuit board or other chassis structure on either side of the blank slot resulting from the removal of a module. In this configuration it is improbably that any foreign object can enter into the recess and short the connector pins. That is, the insertion direction of the module and the possible insertion path of an extraneous object is horizontal when the equipment is being serviced.
However, the physical configuration described above may be inefficient for densely packaged rack mounted chassis as the front-to back chassis dimension is often much greater that an optimum circuit card size. A configuration for a chassis where the modules can be accessed from the top would provide for convenient access to modules interior to the chassis. But, when a chassis is accessible from above, foreign objects may fall into a blank space left by the removal of a circuit card and short pins of the connector.